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Newsgroups: rec.motorcycles
Subject: rec.motorcycles FAQ of 93.02.01 (Part 2 of 6)
Message-ID: <35126.89.uupcb@chaos.lrk.ar.us>
From: dave.williams@chaos.lrk.ar.us (Dave Williams)
Date: 30 Jan 93 20:17:00 GMT
Reply-To: dave.williams@chaos.lrk.ar.us (Dave Williams)
Distribution: world
Organization: The Courts of Chaos * 501-985-0059 * Public Access Usenet
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=====================
Q: What is countersteering, and why would I want to do it?
A: That's a complex subject. Ed Green (egreen@laser.East.Sun.COM)
has submitted a mini-FAQ on the subject:
How do motorcycles turn?
by Ed Green, a recognized non-expert in the field
A frequent topic of discussion, and the source of much confusion,
on rec.motorcycles concerns the mechanics, and underlying physics,
of how motorcycles turn. It would seem that such a basic maneuver
should be well understood by all motorcyclists. Surprisingly, many
experienced riders have no idea how they turn, they probably never
think about it. Many others have very definite ideas about how
they turn, which are likely incorrect. Even among the elite and
erudite Denizens, there is considerable disagreement as to the
mechanics involved with both leaning a motorcycle, and actually
turning one. Outlined here are the four major theories of motorcycle
turning which came out of a recent flame war, er, discussion, on the
subject (hopefully presented somewhat objectively).
To begin with, motorcycles, at greater than parking lot speeds, must
lean to turn. This is part of why they are so fun to ride. The four
predominate theories of turning are evenly divided between leaning
and turning, thusly:
1. Leaning (including countersteering)
1.1 Gyroscopic Precession
1.2 Steering-induced Roll
2. Turning
2.1 Camber Thrust
2.2 Steering Geometry
1. Leaning
Many people assume one leans a motorcycle by shifting body weight.
While it is possible to induce lean in this manner, the quickest,
easiest, and most controllable method of leaning a motorcycle is
by counter-steering. Counter-steering is a technique whereby the
rider exerts pressure on the *inside* handlebar (left for a left-
hand turn, right for a right-hand turn), to lean the motorcycle in
the desired direction. This is in effect steering the motorcycle
the "wrong" way, from the point of view of operating a tricycle,
thus the term "counter-steering." THAT it works is undeniable, and
can be proven easily with a short ride. Both the rate of
transitioning to the lean (how quickly the bike leans) and the
degree of the lean (how far over it leans) can be controlled by
the rate and degree of pressure applied to the grip.
HOW counter-steering works is debatable. The two leading theories
are gyroscopic precession and steering-induced roll.
1.1 Gyroscopic Precession
When riding at any appreciable speed, both wheels of the
motorcycle exhibit the properties of a gyroscope. This
contributes to the stability of a moving motorcycle. One such
property, called gyroscopic precession, is that when a lateral
force is applied to the axis of a free spinning gyroscope, the
axis moves in a direction 90 degrees from the applied force, in
the direction of the spin. For a motorcycle front wheel, this
means that a force applied which attempts to move the axis on one
side of the wheel back, toward the rear of the bike, will result
in an effort by that axis to move up. Similarly, a force applied
to move the hub forward results in a force trying to push it down.
Thus to execute, say, a right hand turn, the rider pushes on the
right grip, seemingly to steer the front wheel to the left. The
forks translate this input to hub of the wheel as a force pushing
forward on the right side of the wheel, and backward on the left
side. Gyroscopic precession will generate a force directed down
on the right side, and up on the left, which forces the bike to
lean to the right.
This gyroscopic business can most easily be demonstrated with a
bicycle. Position the bike in such a way that the front tire is in
the air and can rotate freely, and spin it up to a good speed.
Holding the handlebars, attempt to turn the wheel to the left, by
pushing on the right grip, and pulling on the left. You will feel
the wheel trying to push the whole assembly over to the right.
1.2 Steering-induced Roll
The point of contact between the motorcycle and the road is
called the contact patch, and is that small area of the front and
rear tire beneath the hubs (on level ground) which is resting on
the road surface. All actions which depend on frictional forces
between the tires and the road (such as acceleration, braking,
and cornering) depend on this contact patch, and the contact
patch is the point of application of the frictional forces. As
long as one accepts that the center of mass of the motorcycle+
rider system is somewhere above this contact patch, then any
frictional force which displaces the tires to the side acts at
the point of contact with the road, and will induce a pivot about
the center of mass (with a moment arm of the distance from the
contact patch to the center of mass). This pivot about the
longitudinal axis of the motorcycle is what is referred to in
aviation as roll (when one wing of the plane is higher than the
other).
This steering-induced roll is much easier to visualize than
gyroscopic forces, and is most simply described as "steering the
wheels out from under the bike, to make it lean the other way."
2. Turning
Through one or the other of these techniques, or some combination
of the two, the motorcycle is leaned over by counter-steering.
The motorcycle can also be leaned through weight shift, in which
case neither of the above theories hold. That is only half the
answer, however. Once the motorcycle is leaned, what makes it
turn? Assuming for a moment it doesn't just fall over, why
shouldn't it continue leaned over in a straight line until it
runs out of gas? The good folks of rec.motorcycles have been hard
at work on that one, too. The two leading theories for cornering
of a leaned-over bike are camber thrust and steering geometry.
2.1 Camber Thrust
Unlike the tires for cages and bulldozers, motorcycle tires have
a round profile. The diameter of the tire changes from a maximum
at the center (the contact patch if going straight), decreasing
to a minimum at the rim. When the bike (and thus the tire) are
leaned over, during a single revolution of the wheel, the center
portion of the tire must travel further than the part near the
edge. This induces what is called camber thrust, and it forces
the tire to trace out an arc in the direction of the smaller
diameter.
Camber thrust can be demonstrated by placing an empty ice cream
cone on its side, on a flat surface, and giving it a push. The
cone will not roll in the direction it was pushed, but will
constantly turn from that path, toward the smaller end.
2.2 Steering Geometry
The steering stem and handlebars of a motorcycle are not
positioned directly above the hub of the front wheel, but
are angled back to varying degrees, referred to as rake. Thus,
the weight of the motorcycle+rider is supported by the front
wheel, not straight up and down, but at an angle which points
through to a point ahead of the contact patch. The distance from
that point to the actual contact patch is called trail. During
straight-up and straight-ahead riding, this has no effect on
steering, because the force is transmitted along the center axis
of the motorcycle, which is the same as the center axis of the
front wheel. When leaned over, however, this configuration causes
a force to be applied to the rear part of the front tire, causing
it to lift, turning the wheel into the curve. The motorcycle then
naturally follows the front wheel. This view of turning is
essentially the same as that for other vehicles, using lean only
to counter the centripetal acceleration caused by the bike's
linear momentum.
This aspect of steering geometry can be demonstrated by holding
a bicycle (or motorcycle) upright, with the handlebars and front
tire free to move. Lean the bike to the right, and the front
tire will "fall" (actually, be pushed by the steering geometry)
to the right, and similarly for a lean to the left. Thus, the
lean of the motorcycle, even if initiated by a countersteer,
forces the front tire to point to the inside of the curve,
carving out a turn, and the lean also balances the centripetal
force which would otherwise push the bike over to the outside of
the turn.
So, in summary, a motorcycle must lean to turn. The quickest
way to lean a motorcycle is to use counter-steering. Counter-
steering leans the motorcycle through some combination of
gyroscopic precession and steering-induced roll. Once leaned
over, the motorcycle turns by some combination of camber thrust
and steering geometry. What is the exact combination of each of
these effects in actually causing leaning and turning? Pick
two, three, or all four of them, come up with your own mix, go
find a biker, and argue about it.
*** Mechanical:
Q: I pulled a wheelie the other day. Now my steering has a "catch" in
it, the the bike wants to veer to the side.
A: You probably notched the steering head bearings. Under a side load
(like whopping the front wheel back on the ground) the races can
get dented, balls flattened, or both. The only fix is to replace
the bearings. Taper rollers are much less susceptible to this sort
of damage, and conversion kits are sometimes available.
=====================
Q: My 1978 Suzuki GS750 is supposed to use grade SE oil. All I can
find is SG. Will it work? What about special motorcycle oils
like Golden Spectro?
A: The SE and SG are Service Ratings. SE has been superceded by SG,
which is generally a superior grade of oil. There should be no
troubles running SG.
The vendors of the specialty motorcycle oils claim their oils are
better than ordinary automotive oils. That could possibly be
true, particularly with regard to motorcycles using roller bearing
cranks. In most cases special motorcycle oils are probably a
waste of money.
Some motorcycles never seem to shift right, usually because they
have clutches that run in oil and automotive oil doesn't always
let them properly disengage. Various rec.moto participants have
reported greatly improved shifting and clutch action with special
motorcycle oils. Even expensive oil is cheaper than a clutch or
transmission.
Ed Hackett maintains a detailed FAQ on oil. You can annoy him at
edh@wheeler.wrc.unr.edu.
(end of part 2)